Most radiation curable coating compositions, when exposed to ultraviolet light or electron beam, are cured to glossy, crosslinked coatings. In many instances, however, it is desired to obtain crosslinked coatings of low gloss. One way to achieve low gloss coatings is by adding flatting pigment. Unfortunately, when the amount of flatting pigment is increased to the extent required to produce coatings of very low gloss, the pigment to binder ratio may be so high that there is insufficient binder to firmly hold the pigment. The result is a low gloss coating which lacks hardness and durability. Also, high pigment to binder ratios may increase viscosity of the coating composition to such an extent that application onto a substrate may be hindered.
Another method heretofore employed to produce low gloss films from radiation curable compositions utilized a two step curing process wherein polymerization of certain coating compositions was inhibited in surface portions in the first step by the presence of oxygen (air), and curing of the coating was completed in the second step in an inert atmosphere. Shrinkage of underlying layers during the first step caused pigment particles to be driven into the surface portions, whereby the surface contained a larger amount of pigment than the body of the film which reduced the gloss of the film without sacrificing film strength or rheology properties of the coating composition. U.S. Pat. Nos. 3,918,393 (Hahn) and 4,048,036 (Prucnal) illustrate this approach. A drawback to the approach of concentrating flatting pigments at the surface of the film is that the surface is subject to physical damage that causes unattractive marking of the surface. One form of physical damage is burnishing, that is, the pigment particles can be abraded from the surface, thereby producing an area of higher gloss and creating a non-uniform finish appearance. Another form of damage to which these types of low gloss coatings are susceptible is metal marking, whereby contact of certain metals with the surface causes dark markings. It would be desirable to avoid these problems with radiation curable low gloss coatings as well as to be able to produce coatings of even lower gloss than prior art methods.
Multi-step radiation curing techniques have also been proposed for producing textured finishes. These techniques are disclosed in U.S. Pat. Nos. 4,421,784 (Troue), 3,840,448 (Osborne et al.), and 4,411,931 (Duong). The methods of these patents do not appear to be intended to produce the type of flat, low gloss finishes that are the subject of this invention, but are directed to the production of relatively gross, visibly perceivable, surface wrinkle patterns. These patented methods do not involve the use of electron beam radiation.
Prior art techniques for producing low gloss or textured radiation cured coatings, including the patents set forth above, have generally been limited to non-pigmented coatings. Typically the low gloss or textured coating is a clear coating that is applied over a pigmented base coat. This has generally been considered necessary in order to assure adequate penetration of radiation during the curing step. It would be highly desirable to produce low gloss coatings by radiation curing directly from pigmented coating compositions.
Commercially available photoinitiators such as 2-chlorothioxanthone are capable of producing low gloss pigmented coatings, but have the undesirable drawback of yielding reaction by-products that impart yellow coloration to the cured coating. This causes uncontrolled color shifting, which is particularly objectionable in white or other light colored coatings.